The present invention relates to a method for water purification with a water-specific reaction, and, more particularly, to a method and apparatus for purification of water contaminated by organic and/or inorganic compounds (petroleum; nuclear, biological, chemical (NBC) agents; and other volatile compounds) by trapping water in an anhydrous salt and then isolating water by decomposing the hydrate and condensing the purified water vapor while, at the same time, regenerating the desiccant.
In the past, it was known that water could be extracted from the atmosphere by using physical adsorbents such as silica gel or molecular sieves which are inert and have been used with no reported hazards. Practical apparatus can be constructed for extracting 200 gallons of water per hour at 80.degree. F. and 10% relative humidity. Higher relative humidities yield higher water production, e.g. 700 gallons per hour at 40% RH and 80.degree. F.; 1000 gallons per hour at 55% RH and 80.degree. F. It was not, however, considered that such devices could be used for water purification.
Several methods are also known to remove water from the atmosphere. These include the use of refrigeration cycles and the passing of water vapor through a membrane. The most energy-efficient technique is, however, to use a desiccant to adsorb the water from the atmosphere, which would then be recycled to release the water and regenerate the desiccant. Known water purification methods depend on the quantity and nature of the contaminant. For example, if the level of contamination was more than that of treatment for microorganisms, such as chlorination, then some form of separation was used. Most known methods used for liquid-phase water purification require that the contaminant be removed from water. This type of separation of the impurity has been effected by filtration, adsorption, coagulation and sedimentation, or ion-exchange.
Water which has been contaminated by volatile compounds is difficult to purify by conventional methods such as distillation, reverse osmosis or ultrafiltration. Adsorbents might be used to remove petroleum or other contaminants from water but they have limited selectivity and limited capacity. Therefore, the use of activated carbon or similar adsorbents to remove gross contamination would not be practical. Distillation will not separate a volatile component from water, since both would distill. Reverse osmosis and ultrafiltration will also have difficulty in removing petroleum or other volatile contaminants. None of these conventional methods would be effective for NBC (nuclear, biological or chemical) contamination.
Using an adsorption principle, however, with a water-specific reaction, the present invention permits effective and efficient water purification by using air saturated with water vapor from the contaminated water source. Only water from the vapor stream is extracted by the desiccant. Contaminates in the water vapor pass through the system without reacting with a desiccant in the form of an anhydrous salt; pure water is obtained by dehydration while the air stream containing the contaminates exits the system.
In carrying out this purification process, a presently contemplated embodiment of the present invention utilizes a desiccant bed comprised of a suitable anhydrous salt to effect water extraction at low relative humidities. A suitable salt must have a rate of adsorption which is much higher than other desiccants, must be non-toxic and should be recyclable many times.
Another aspect of the system in accordance with the present invention is that it can operate at the minimum relative humidity and stay within the weight and size limitations with the use of a low pressure blower (i.e. about 20 inches of water pressure). Typically, such a blower powered by a Diesel engine will have a capacity of 150,000 CFM. This blower will be incorporated in a system having a weight of 14,500 lbs (not including desiccant), a desiccant consisting of anhydrous salt, a desiccant bed area of 75 ft.sup.2 and a 1 hour cycle period.
As is well known, the water content of air is a function of temperature. As the temperature increases, the amount of water that air can hold increases. Relative humidity is the ratio of the quantity of water in air to the maximum quantity of water air can hold at a given temperature. Therefore, if the air temperature is increased, the total amount of water in air at a fixed relative humidity would be increased. The size of the system is controlled by the weight and size limitations of the hardware, but the amount of water that can be held by the system is determined by the quantity of the desiccant in the adsorption bed. The limits on collection for the high relative humidities would be the size of the bed, energy available, and recycle time.
Thus, it has been found possible to construct a water collection device that will produce purified water by separating volatile organic and inorganic contaminants at 200 gal/hr from air with a relative humidity of 10% and 80.degree. F. It would require two of these units to produce purified water at 200 gal/hr and 5% RH at 80.degree. F. If the relative humidity is raised to 40% and 80.degree. F., the volume of purified water collected per hour will increase to 750 gallons, and at 55% RH and 80.degree. F. the collection rate would be 1000 gal/hr. The system has the additional advantage of being simple, rugged and self-contained.
The following hydrates have been identified as particularly suitable for the water purification apparatus and method of the present invention based on low toxicity, low dehydration temperature, and high percent of water in the hydrated salt: cupric sulfate pentahydrate; sodium acetate trihydrate; cadmium sulfate octahydrate; ferrous sulfate heptahydrate; cobalt chloride hexahydrate; aluminum sulfate octadecahydrate; and zinc sulfate heptahydrate.
For a desiccant-anhydrous salt to perform properly in the present invention, the decomposition temperature of the hydrate should be above the atmospheric boiling point of water. This would permit distillation of the water from the desiccant to a condenser at atmospheric pressure. In addition, if the desiccant loses water below the boiling point of water, then the water will dissolve part or all of the salt and thus lead to caking of the desiccant.
In a process embodying the present invention, the first step causes the contaminated water vapor to saturate the air at the inlet of the system which contains an anhydrous salt-desiccant bed. The saturated air is then forced through the bed, but only the water reacts with the salt to form a hydrate. The contaminates will pass through the system and have no interaction with the bed. The second step is to heat and decompose the hydrate by removing (condensing) the water to provide a source of purified water in a simple yet effective manner.
In one presently contemplated embodiment of a system in accordance with the present invention, three desiccant beds are used. Air is forced into a manifold where it is valved through one of the three desiccant beds. The exit air, which is dehydrated, it is passed into the second bed, which is initially hot from a previous regeneration step. This second bed is cooled for use as the next bed to hydrate. A third bed is in the process of being regenerated by distilling the trapped water through the condenser. Finally, water is collected in a receiver. At the end of this cycle, the first bed is hydrated, the second bed is cooled, and the third bed is hot but dehydrated. There are three steps in the process for hydrating and dehydrating the desiccant beds. Heat from the Diesel engine exhaust may be used to dehydrate the beds, and air-cooled condensers are used to collect the water vapor.